I need help finding the second derivative and critical numbers of this function.

$\displaystyle f(x)=x^{2/3}(x-3)^{1/3}$

I get a first derivative of

$\displaystyle f'(x)=2/9x^{1/3}(x-3)^{1/3}+1/3x^{2/3}$

and a second derivative of

$\displaystyle f"(x)=(x-3)^{1/3}+{2/9}x^{1/3}$

but i dont think this is right because I cant find the critical numbers